1. Field of the Invention
The present invention relates a secondary balancing device (balancer) of an outboard motor vertical type engine and the engine provided with such a secondary balancer.
2. The Related Art
A four-stroke-cycle engine is susceptible to vibration, which is caused by a reciprocating motion of a piston of a cylinder assembly of the engine. For the purpose of eliminating the vibration, a certain type of engine is provided with a balancer or balancing device (called “balancer” hereinlater). Such a balancer includes a balancer shaft with a balancer weight corresponding to the total mass of movable parts such as piston and connection rod of the engine so that rotation of the balancer shaft in conjunction with operation of the engine eliminates the vibration caused by the operation of the engine.
The above-mentioned balancer may include a primary balancer, to eliminate primary vibration, in which a vibration exciting force generates once during one turn of the crankshaft, and a secondary balancer, to eliminate a secondary vibration, in which a vibration exciting force generates twice during one turn of the crankshaft.
For example, an engine for an outboard motor is mounted in a state that a crankshaft is kept in the vertical direction, and the other structural components such as a crankcase, a cylinder block and a cylinder head are combined together so as to constitute the engine, i.e., vertical in-line engine.
With an arrangement of the secondary balancer in such an engine, Japanese Laid-Open Patent Publication No. HEI 10-184798 discloses a secondary balancer in which a right-hand secondary balancer shaft and a left-hand secondary balance shaft are arranged vertically on the opposite side surfaces of a cylinder block so that four cylinders formed in the cylinder block are located between these balancer shafts.
In the case of the dual shaft type-balancer, it is necessary to rotate the balancer shafts in the opposite directions to each other so as to eliminate the vibration exciting force in the perpendicular direction to the axial direction of the cylinder. However, in an arrangement in which the balancer shafts are disposed separately from each other on the opposite side surfaces of the cylinder block, the distance between the balancer shafts and the crankshaft, which serves as a driving source for rotating them, is widened, thus leading to complicated structures in the layout of the balancer driving system. Consequently, the number of the structural parts increases, resulting in the increased number of assembling steps and an increased total weight, thus being inconvenient and disadvantageous.
In addition, it is necessary to provide a housing for receiving and rotatably supporting the balancer shafts on the opposite sides of the cylinder block. Accordingly, the existing cylinder block cannot be used as it is, and it is required to manufacture a newly suitable cylinder block. As a result, the cost, especially, a capital investment considerably increases.
Furthermore, since the housings for the balancer shafts project from the side surfaces of the cylinder block, it is required to review the layout of peripherals. The outboard motor includes, to achieve requirement for miniaturization, an intake system, an exhaust system and a fuel system, as well as electrical components, which are put together around the engine. It may be further difficult to make a change in layout of the outboard motor.